Some types of drug therapy are best administered continuously, rather than at fixed intervals. Ambulatory drug delivery provides patients with continuous drug therapy without hospitalization or otherwise being immobilized.
Measured drug therapy utilizes some combination of the following three options: continuous infusion, patient activated demand doses, and programmed automatic doses. For example, insulin may be delivered to a diabetic at a low continuous rate throughout the day, with programmed automatic dose increases proximate to mealtime. In another example, pain medication may be delivered to a patient at a continuous rate throughout the day, while allowing patient activated demand doses as needed. However, because over-delivery or under-delivery of the selected medication may be fatal to the patient, it is essential that the infusion pump accurately determines the amount of medication due to the patient on a real-time basis.
As a practical matter, infusion pumps deliver a fixed quantity of fluid, known as a stroke volume, for each pump activation. The stroke volume corresponds to the minimum quantity of fluid the pump can deliver for each activation. Only whole multiples of the stroke volume can be delivered. Fluid delivery is typically measured by determining the amount of fluid delivered per pump activation (stroke volume) and adjusting the interval between pump activations to alter the rate of fluid delivery.
For purposes of measured drug therapy, the time interval between pump activations can be several minutes. Accurate and timely drug delivery is complicated when a demand or automatic dose is called for between pump activations. In particular, the quantity of medication due for the period from the last pump activation to the demand or automatic dose must be calculated and incorporated into the new delivery rate. However, this quantity of medication due is seldom an even multiple of the stroke volume of the pump. Generally, the undelivered quantity of fluid comprises some even multiple of stroke volumes, plus some remainder which is less than the minimum amount of medication that can be delivered by one pump activation.
Another difficulty with ambulatory drug therapy is that the therapy is most effective when the patient has some degree of control and responsibility for the treatment. Therefore, a programmable drug infusion pump requires various safety features to maximize the flexibility to the patient without allowing the patient to alter operation parameters in a manner which could be dangerous or counter-productive to the drug therapy.